The invention concerns an optical recording medium with a transparent-substrate layer and a method of manufacturing a medium of that type.
The term "optical recording medium" as used herein is to be understood as a medium carrying recorded data or being a blank. If the medium has already recorded data, it can be rerecorded or overwritten. If the medium is blank, on the other hand, data can be recorded thereon.
One known example of an optical recording medium is a compact disc, which has a transparent layer on top of a reflecting aluminum layer. The light-reflecting aluminum layer has depressions called "pits" that represent data stored on the disc. The data can be read from the compact disc by means of an optical pick-up, because the reflectivity of the light-reflecting aluminum layer depends on the pattern of the depressions in the disc. Less light is reflected from a depression, which is often called a "groove," than from an elevation, which is often called "land."
From the intensity of the light reflected by the disc accordingly, the optical pick-up determines whether the bit being scanned is a logical one or a logical zero for example.
Another optical medium of this type, called an "optomagnetic disc," is described in the article "Magnetooptische Versuche dauern an" ["Optomagnetic testing continues"] on pages 37 to 41 of Funkschau 13, 21 (June 1986).
An optomagnetic disc, in contrast to a conventional compact disc, has sometimes no pits. Below the transparent layer is a magnetic layer, in which data can be stored and from which data can be read out. The procedure for writing data onto an optomagnetic disc will now be described.
A laser beam focused on the disc heats the magnetic layer to the vicinity of the Curie point. It is, however, usually sufficient to heat the layer to a compensation temperature that is below the Curie point. An electromagnet is positioned behind the laser beam's focus on the disc and magnetizes the area heated by the laser in one polarity or another. As the temperatures of the heated points drop below the Curie point again when the laser beam is turned off, the magnetic polarity established by the electromagnet remains constant. The individual bits are thereby stored as domains of differing magnetic polarity, with one polarity, for example, representing a logical one and the other a logical zero.
The data can be read out by exploiting the Kerr effect. The plane of polarity of a linearly polarized beam of light is rotated through a measurable angle when reflected by a magnetized domain. Depending on the direction in which the domain is magnetized, the plane of polarization of the reflected beam will be rotated either left or right. Since, however, the recorded domains on the disc act like magnetized mirrors, the plane of polarization of a scanning beam of light will be rotated right or left to a measurable extent depending on the magnetic polarity of the domains being scanned at that instant.
From the rotation of the plane of polarization of the light beam reflected from the disc, the optical pick-up determines whether the bit is a logical one or a logical zero. In contrast to a compact disc, an optomagnetic disc can be erased and rerecorded almost as often as desired.
Although the storage capacity of both types of optical recording media--compact discs and optomagnetic discs--is satisfyingly high, a substantial increase in that capacity would be very desirable and useful, especially in relation to computers and videodisc players.
Japanese Application 62 60 147 discloses an optomagnetic disc with short patterns of pits containing address labels at prescribed intervals along the data-storage tracks. The purpose of the address labels is to allow the data stored on the disc to be arranged in individual sections.
Japanese Application 58 114 343 discloses a circular optomagnetic disc divided into several sectors. The individual sectors store alternately optomagnetic data and data represented by patterns of pits so that every optomagnetic sector is followed by a sector with patterns of pits.
An optomagnetic disc known from Japanese Application 58 211 346 has patterns of pits distributed along the tracks between each pair of optomagnetic layers. One pattern of pits follows each optomagnetic layer as the track is traversed. The patterns of pits represent track addresses.
Finally, Japanese Application 61 68 472 discloses an optomagnetic disc on which the data are stored alternately in spiral and circular optomagnetic data-storage tracks and in data-storage tracks represented by pits. One winding of the spiral or one concentric circle is optomagnetic and the next contains patterns of pits. Thus, one winding or one circle is always situated between two circles or two windings.
None of the aforesaid optomagnetic discs, however, has a storage capacity that is increased by the procedures disclosed in the Japanese applications in relation to that of a conventional optomagnetic or optical disc.